The Black River (also known as the Da River, s?ng ?? in Vietnamese and Lixian River in China) is a river located in China and northwestern Vietnam. Its source is in Yunnan Province of China. It runs through the Vietnamese provinces of Lai Ch?u (where it forms part of the border with ?i?n Bi?n Province), S?n La and H?a B?nh. It is the most important tributary of the Red River, which it joins in Tam Nong District near Vi?t Tr? in Ph? Th? Province. It also forms part of the border between Ph? Th? Province and Hanoi (previously the border with H? T?y Province). The rivers total length is 910 km, around 400 km in China and 527 km in Vietnam.